1. Technical Field
The present invention relates to polyimide resins which have improved rheology during molding and higher physical strength in their final molded product forms. These improvements are due to the higher density of the polyimide resins and the absence of reaction side products and decomposition products.
2. Description of Background Art
As a group, polyimides are very important and useful materials because of their chemical inertness, strength, and temperature resistance. A certain subgroup of the polyimides has additional advantages such as the retention of strength, resiliency and flexibility from cryogenic temperatures to 600.degree. F.; resistance to hydrolysis and ultraviolet degradation; and the capability of undergoing oxidative degradation without generating toxic products or smoke.
This subgroup of polyimides is prepared by reacting an aromatic tetracarboxylic acid compound, e.g., dianhydrides or diester-diacids of 3,3',4,4'-benzophenone tetracarboxylic acid with one or more primary aromatic diamines to form a polyamide-acid. This material is then converted to a polyimide by heating. The polyamide-acid may also contain low molecular weight end caps, e.g., norbornene, substituted norbornene, or their ester counterparts. When polymerizable end caps are used, the application of heat and pressure not only converts the polyamide-acid to a polyimide but effects crosslinking among the polyimides to form a thermoset resin.
High pressure (compression) and low pressure (autoclave) methods have been developed for the final treatment of polyimide impregnated composites, as well as pure polyimide molded articles. It has been found that the single greatest adverse impact on the physical and mechanical strength of the product is the formation of voids during this final treatment. Several references have traced the source of the voids to residual solvents remaining during fabrication or to the presence of tri and tetra esters of the tetracarboxylic acid compound, e.g., 3,3',4,4'-benzophenone tetracarboxylic dianhydride (BTDA), in the monomer solution (Lauver et al. NASA TM-79068 (1979)). Accordingly, tri- and tetra-esters of the tetracarboxylic acid compound are avoided in the synthesis of the polyamide-acid intermediate. Additionally, carefully controlled heating during compression molding or autoclave processes has been used to give relatively void free composites.
The present inventor has found another significant source of voids which has not been appreciated by those skilled in the art. The prior art discloses the reaction of the dimethylester of 3,3',4,4'-benzophenone tetracarboxylic dianhydride (BTDE), methylene dianiline (MDA) and norbornene ester (NE) to form high temperature thermoset polyimides (PMR). As generally taught in the art, the three monomers are simultaneously mixed and reacted to form PMR. This leads to the kinetically favored reaction of MDA with NE, rather than BTDE, to form undesirable side products which are no longer capable of oligomerization to form the polyamide-acid, as well as unreacted BTDE monomer. Additionally, this reaction depletes the amount of NE for end-capping. Thus, a significant amount of the polyamide-acid molecules formed by the reaction of BTDE and MDA retain unreacted methylester terminal groups. These terminal groups subsequently form anhydride terminal groups which are susceptible to decomposition and release of CO and CO.sub.2 during molding, curing and use of the final resin product. The release of these gaseous products from the decomposition of the anhydride terminal groups leads to bubbles and voids within the resin which significantly and adversely affect the resin rheology and physical strength.
For the above reasons, there is a need in the art for an improved process for the manufacture of high temperature thermoset polyimides of low anhydride content.
Accordingly, it is an object of the present invention to provide polyimides for composites and molded articles that are free of voids resulting from the decomposition of anhydride groups. The lower viscosity also allows more intricately molded parts to be made without voids.
Another object is to provide novel processes for making the aforementioned improved polyimides.
These and other objects of the invention, as well as a fuller understanding of the advantages thereof, can be had by reference to the following description and claims.